How some snakes became venomous

This 2017 video from Australia says about itself:

The inland taipan and eastern brown snake are the 2 most venomous snakes in the world.

From Swansea University in Wales:

Snake venom evolved for prey not protection

March 25, 2020

It is estimated that every year, over 100,000 human deaths can be attributed to snakebite from the world’s 700 venomous snake species — all inflicted in self-defence when the snakes feel threatened by encroaching humans.

However, a new piece of research concludes that snake venom did not evolve as a defence mechanism.

Senior Lecturer Dr Wolfgang Wüster, an expert on snake venom evolution at Bangor University’s School of Natural Sciences explained:

“We know that snake venom is used primarily for foraging; for overpowering and killing prey. However, we also know that snakes use their venom in self-defence — that’s why so many people get bitten, and sometimes killed, by venomous snakes worldwide. We wanted to investigate whether defence was a driver in venom evolution.”

To provide efficient defence from a predator, the snake venom would need to provide sufficient instant pain to deter the predator and enable the snake to escape, much as a bee-sting hurts immediately.

A new paper published in Toxins reveals that surprisingly few venomous snake bites cause immediate pain, implying that the venom make-up has not evolved for a defensive primary purpose.

Bangor University student Harry Ward-Smith tested this hypothesis under Dr Wüster’s supervision.

He gathered online survey responses from reptile keepers, herpetologists and fieldworkers, as part of his undergraduate degree in Zoology with Herpetology.

His survey, which gained nearly 400 responses world-wide, asked people working with snakes about the time-course of pain after bites they had sustained.

The results of around 600 experiences revealed that only a minority of bites and species regularly cause rapid, severe pain. Moreover, where early pain evolves during snake evolution, it is likely to be lost again in descendant lineages.

Dr Kevin Arbuckle, Senior Lecturer in Biosciences (Evolutionary Biology) at Swansea University’s Department of Biosciences and a co-author of the paper commented:

“Our results suggest little evidence for widespread evolution of venoms driven by their use in defence, though interesting exceptions likely exist such as the defensive use of venom ‘spitting’ in some cobras, and these specific cases deserve further study.”

Dr Wüster added: “Even though we might have expected defending your life to be more important than feeding, it turns out that natural selection for diet does seem to be the main driver of venom evolution in snakes”.